Idaho alfalfa growers are warned to watch for blister beetle infestations after state ag department officials confirmed an outbreak of the poisonous insects in the foothills area of Ada County near Boise.

Seldom a problem in the Pacific Northwest, blister beetles contain cantharidin, a toxic chemical that can be deadly to cattle, sheep and especially horses. Beetle larvae feed on grasshopper eggs, but the adults eat vegetation, occasionally moving to flowering fields such as alfalfa, say University of Idaho Extension entomologists.

Blister beetles have long, narrow bodies, broad heads, and antennae that are about one-third the length of their bodies. The front wings are soft and flexible in contrast to the hard front wings of most beetles. The punctate blister beetle identified in Idaho is black, while other species are tan to gray.

In alfalfa fields, blister beetles usually congregate, or swarm, in small areas, typically within 50 yards of field margins. Since dead beetles are as poisonous as live ones, experts say it’s important to keep them out of hay. One way is to mow without conditioning, which enables the beetles to leave the windrows and fly away. Applying an insecticide, especially around field margins, is another option.

The Idaho entomologists are asking alfalfa growers in that state to contact their local Extension offices for beetle management information. For more on blister beetles, read “Keep Blister Beetles Out Of Hay” and “Battle Blister Beetles.”